Bump jekyll to v3.9.4

[parkr]

Adds Ruby 3.3 support. (https://github.com/jekyll/jekyll/releases/tag/v3.9.4)

@yoannchaudet, if you'd like to bring support up to Ruby 3.3, this tiny v3.9.4 release has been made available for you!

[parkr]

That's odd. The tests in spec didn't run.

[dorianmariecom]

+1 I was getting:

14:38:58 web.1  | jekyll 3.9.3 | Error:  undefined method `[]' for nil
14:38:58 web.1  | /Users/dorianmariefr/.asdf/installs/ruby/3.3.0/lib/ruby/3.3.0/logger.rb:384:in `level': undefined method `[]' for nil (NoMethodError)
14:38:58 web.1  | 
14:38:58 web.1  |     @level_override[Fiber.current] || @level
14:38:58 web.1  |                    ^^^^^^^^^^^^^^^
14:38:58 web.1  | 	from /Users/dorianmariefr/.asdf/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/jekyll-3.9.3/lib/jekyll/log_adapter.rb:43:in `adjust_verbosity'
14:38:58 web.1  | 	from /Users/dorianmariefr/.asdf/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/jekyll-3.9.3/lib/jekyll/configuration.rb:157:in `config_files'
14:38:58 web.1  | 	from /Users/dorianmariefr/.asdf/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/jekyll-3.9.3/lib/jekyll.rb:113:in `configuration'

[yoannchaudet]

This made it into #898.